If you mean you want to draw a rectangle with rounded corners, you can use the rounded rectangle tool. Set your color before you start, then click-hold or right click on the custom shape tool and choose the rounded rectangle tool. If that's not what you mean, can you explain in more detail or show an example of what you want to do?

While you can create a retangular selection (dancing ants), there isn't a method for bending the lines. You can transform a selection you have created by rotating it, scaling it, or skewing it (giving it a lean). You can use other selection tools to modify your existing selection, including adding to, subtracting from, finding the intersection of, or the exclusion of. You can use these combinations with a Elliptical selection to give your rectangular selection bent lines. Photoshop proper, the parent product, does have warp controls for transformation which would make such as task easier.

Once you have a selection you like, you can fill it with color.

You can get a reduced price for Elements 11 because you own Elements 10.

You can kind of bend lines on a rectangular selection by adding a feather (which can make your selection look like it has rounded corners), but if you're planning to fill with color anyway, it's much simpler just to use the rounded rectangle tool.